1. Field of the Invention
The present invention relates to a light emitting material in which generated electroluminescence is obtained by imparting an electric field, and to a light emitting device using the light emitting material. In particular, the present invention relates to a light emitting device using an organic compound in the light emitting material.
2. Description of the Related Art
Display devices using liquid crystals have a typical construction which uses a back light or a front light, and they display an image using that light. Liquid crystal display devices are employed as image display means in various types of electronic devices, but they have a structural disadvantage in that their angle of view is narrow. In contrast, display devices using light emitting materials in which electroluminescence is obtained have a wide angle of view, and are the focus of next generation display devices due to their superior visibility.
Light emitting elements using organic compounds as light emitting materials (hereafter referred to as organic light emitting elements) are structured by appropriately combining layers such as hole injecting layers, hole transporting layers, light emitting layers, electron transporting layers, and electron injecting layers, formed by organic compounds, between a cathode and an anode. Although hole injecting layers and hole transporting layers are represented here as being distinct, they are the same in that hole transportability (hole mobility) is their particularly important property. In order to make distinction convenient, the hole injecting layer is taken as the layer on the side contacting the anode, and the hole transporting layer is taken as the layer on the side contacting the light emitting layer. Also, the electron injecting layer is taken as the layer containing the cathode, and the electron transporting layer is taken as the layer on the side containing the light emitting layer. There are times when the light emitting layer also serves as an electron transporting layer, and therefore it is also referred to as a light emitting electron transporting layer. Light emitting elements formed by combining these types of layers show rectification characteristics, and have structures similar to diodes.
It is thought that structures that emit light by electroluminescence do so by a phenomenon in which electrons injected from the cathode and holes injected from the anode recombine in the layer made from the light emitting material (light emitting layer), forming excitons, and light is irradiated when the excitons return to the ground state. Fluorescence and phosphorescence exist as types of electroluminescence, and these are understood as light emission from a singlet state (fluorescence) and light emission from a triplet state (phosphorescence) in the excited state. The brightness of the light emitted reaches from several thousands to several tens of thousands of cd/m2, and therefore it is considered possible to apply electroluminescence in theory to display devices and the like. However, various types of degradation phenomena also exist, and several problems to their practical application remain.
It is thought that there are five causes of degradation to light emitting materials made from organic compounds, and to organic light emitting elements: 1) chemical degradation of the organic compound (via the excited state); 2) melting of the organic compound due to heat generated during driving; 3) dielectric breakdown originating in macro faults; 4) degradation of the electrodes or electrode/organic layer interfaces; and 5) degradation due to instabilities in the amorphous structure of the organic compound.
The above first to third causes of degradation are due to driving of the organic light emitting element. The generation of heat is inevitable because electric current flowing within the element is converted into joule heat. If the melting point of the organic compound, or the glass transition temperature, is low, it is thought that melting will occur. Further, the existence of pinholes or cracks within the organic compound will concentrate the electric field in those locations and cause dielectric breakdown. Degradation proceeds even if the light emitting element is maintained at room temperature with the fourth and the fifth causes. The fourth cause is known as dark spots, and is due to cathode oxidation and reactions with moisture. The fifth cause is due to the fact that all organic compounds used in the organic light emitting element are amorphous materials. It is thought that crystallization occurs during long term storage, changes by long time, and the generation of heat, and that there are almost no materials with a stable amorphous structure can be maintained.
Dark spots can be well suppressed by using better sealing techniques, but in practice, degradation is caused by a composite of the aforementioned reasons, and the situation is difficult in order to be uniformly understood. A method in which organic light emitting elements formed on a substrate are sealed airtight by a sealing material, and a drying agent is formed on the inside space, is known as a typical sealing technique. However, the phenomenon in which the electric current flowing in the organic light emitting elements decreases, along with a decrease in the brightness of emitted light, if a constant voltage is continually applied to the light emitting element is thought to originate in the physical properties of the organic compound.
Both low molecular weight organic compounds and high molecular weight organic compounds are known as organic compounds for forming organic light emitting elements. Compounds such as the following are known as examples of low molecular weight organic compounds: xc3xa1-NPD (4,4xe2x80x2-bis(N-(1-naphthyl)-N-phenylamino)-biphenyl), which is a copper phthalocyanine (CuPc) aromatic amine-based material, and MTDATA (4,4xe2x80x2,4xe2x80x3-tris(N-3-methylphenyl-N-phenylamino)triphenylamine) as hole injecting layers; and tris-8-quinolinolate aluminum complex(Alq3) as a light emitting layer. Among the high molecular weight organic light emitting materials, examples such as polyaniline and polythiophene derivatives (PEDOT) are known.
Looking at the diversity of materials, low molecular weight organic compounds manufactured by evaporation have a remarkable diversity compared to high molecular weight organic materials. However, whatever material is used, organic materials which can be made purely from only basic structural units are rare. Different kinds of bonds and impurities are mixed in by manufacturing processes, and there are also times when various additives such as pigments are added. Further, these types of materials include those which deteriorate due to moisture and which are easily oxidized. It is possible for moisture, oxygen, and the like to easily become mixed in from the atmosphere, and therefore caution is required in material handling.
It is known that when organic compounds are degraded by light, chemical bonds become double bonds and the structure changes to one containing oxygen (such as xe2x80x94OH, xe2x80x94OOH,  greater than Cxe2x95x90O, xe2x80x94COOH). It is therefore thought that the bonding state changes and degradation advances if the organic compounds are placed within an atmosphere containing oxygen, or if oxygen or H2O exist as impurities within the organic compounds.
Seen as a diode, one type of semiconductor element, it is known that impurities caused by oxygen form localized levels within the forbidden bands in semiconductor elements having semiconductor junctions, and that this causes junction leakage and drops in the carrier lifetime, thereby greatly reducing the semiconductor element properties.
FIG. 11 is a graph showing the distribution in the depth direction of oxygen (O), nitrogen (N), hydrogen (H), silicon (Si), and copper (Cu), measured by secondary ion mass spectroscopy (SIMS), in an organic light emitting element. The structure of the sample used in making the measurements is as follows: tris-8-quinolinolate aluminum complex (Alq3)/carbazole-based material (Ir(ppy)3+CBP)/copper phthalocyanine (CuPc)/oxide conductive material (ITO)/glass substrate. Alq3 contains oxygen within its molecular structure, as shown by the chemical formula (Chem 1) below. 
On the other hand, Ir(ppy)3+CBP and CuPc are structured such that there is no oxygen contained within their molecules, as shown by the following chemical formulae (Chem 2 and Chem 3): 
The oxygen concentration becomes high in the Alq3 region and in a certain part of the ITO region in the concentration distributions of each of the elements shown by FIG. 11 for this reason. Conversely, the oxygen concentration is reduced in the Ir(ppy)3+CBP and CuPc layers. However, ions are detected on the order of 7xc3x97102 counts/sec, and it can be confirmed that oxygen exists in regions at which oxygen is not expected to be.
The highest occupied molecular orbital (HOMO) level degenerates and therefore oxygen molecules are unique triplet state molecules at their base state. Normally, the process from triplet to singlet excitation becomes more difficult to occur due to forbidden transitions (spin prohibitions), and therefore singlet state oxygen molecules are not generated. However, if triplet excitation state molecules (3M*) having an energy state that is higher than the singlet state exist in the periphery of the oxygen molecules, then this can lead to a reaction in which singlet state oxygen molecules are generated, in accordance with energy transfers occurring as shown below.
3M*+3O2xe2x86x92M+1O2xe2x80x83xe2x80x83Eq 1 
Seventy-five percent of the molecular excitation state in the light emitting layers of organic light emitting elements is said to be triplet state. Therefore, if oxygen molecules are mixed within the organic light emitting elements, then the generation of singlet state oxygen molecules can be obtained by the energy transfer of Eq 1. Singlet excitation state oxygen molecules have ionic nature (bias to electric charge), and therefore it is thought that there is the possibility of reaction with the electric charge bias developing in the organic compound.
For example, methyl group in batho-cuproene (hereafter referred to as BCP) is an electron donor, and therefore carbon directly bonded to conjugate rings is positively electrified. As shown by Chem 4 below, singlet oxygen with ionic properties reacts with carbon which is positively electrified, and there is the possibility that carboxylic acid and hydrogen are formed as shown by Chem 5 below. As a result, it can be expected that the electron transportability will decrease. 
Based on these considerations, it has been found that impurities such as oxygen and H2O contained within organic compounds are impurities which cause a variety of types of degradation such as a reduction in brightness in organic light emitting elements and in organic light emitting devices using the organic light emitting elements.
A first object of the present invention is to reduce the concentration of oxygen, which causes electrode material degradation such as reductions in brightness and dark spots, in organic light emitting elements having layers made from organic compounds between a cathode and an electrode, and in light emitting devices structured by using the organic light emitting elements.
A desirable applied example using organic light emitting elements is an active matrix drive light emitting device in which a pixel portion is formed by the organic light emitting elements. A thin film transistor (hereafter referred to as TFT) is formed in each pixel as an active element. It is known that the values of the properties of TFTs formed using semiconductor films, such as the threshold voltage, fluctuate due to alkaline metal contamination. A second object of the present invention is to provide an appropriate structure for forming a pixel portion by combining organic light emitting elements, which use alkaline metals having small work coefficients in their cathodes, with TFTs.
With the present invention, impurities which contain oxygen, such as oxygen and H2O, contained within organic compounds used for forming an organic light emitting element are reduced in order to prevent degradation of a light emitting device. Oxygen, hydrogen, and the like are of course contained as structural elements with organic compounds, and the term impurities corresponding to organic compounds refers to exogenous impurities not contained within the conventional molecular structure in the present invention. These types of impurities are expected to be present within organic compounds as atoms, molecules, free radicals, and oligomers.
In addition, the present invention has a structure for preventing problems such as fluctuations of the threshold voltage due to alkaline metals, such as sodium and potassium, contaminating the TFTs in an active matrix drive light emitting device.
With the present invention, such impurities are eliminated, and the average concentration of impurities contained in layers made from organic compounds which is used in order to form organic light emitting elements such as hole injecting layers, hole transporting layers, light emitting layers, electron transporting layers, and electron injecting layers, is reduced to be less than or equal to 5xc3x971019/cm2, preferably less than or equal to 1xc3x971019/cm2. In particular, it is necessary to reduce the oxygen concentration in the light emitting layer and its vicinity.
When an organic light emitting element emits light at a brightness of 1000 Cd/cm2, this corresponds to an emission amount of 1016 photons/sec-cm2 when converted. If the quantum efficiency of the organic light emitting element is assumed to be 1%, then it is necessary to have an electric current density of 100 mA/cm2. In accordance with an empirical rule based on semiconductor elements such as solar batteries and photodiodes using amorphous semiconductors, it is necessary to have a defect level concentration equal to or less than 1016/cm3 in order to obtain good characteristics for elements in which this order of electric current is flowing. In order to achieve this value, it is necessary that the concentration of harmful impurity elements forming the defect level should be reduced to an amount less than or equal to 5xc3x971019/cm2, preferably less than or equal to 1xc3x971019/cm2 as above.
An apparatus for forming organic compounds used in making organic light emitting elements, and for reducing the organic compound impurities, is structured as follows.
The side walls on the inside of a film formation chamber in an evaporation apparatus for forming layers made from low molecular weight organic compounds are given a mirrored surface by electrolytic polishing, reducing the amount of gas emission. The material used in the film formation chamber is stainless steel or aluminum. Heaters are formed on the outside of the film formation chamber for the purpose of preventing gas emission from the inside walls, and a baking process is performed. Gas emissions can be greatly reduced by the baking process, and conversely, it is preferable to perform cooling by using a refrigerant during evaporation. A turbo molecular pump and a dry pump are used in the evacuation system, preventing reverse diffusion of oil vapor from the evacuation system. Further, a cryo-pump may also be used in conjunction with the other pumps in order to eliminate any remaining H2O.
The evaporation source is based on a resistance heating type, but a Knudsen cell may also be used. The evaporation material is introduced into the film formation chamber from an exchange chamber of load lock type attached to the film formation chamber. Exposure of the film formation chamber to the atmosphere is thus prevented as much as possible during evaporation of the evaporation material. The evaporation source is mainly an organic material, and sublimation purification is performed within the film formation chamber before evaporation. Further, a zone purification method may also be applied.
Preprocessing of a substrate introduced to the film formation chamber may be by gas emission processing performed by heat treatment, or by plasma processing using argon. Impurities emitted from the substrate are reduced as much as possible. TFTs are already formed on the substrate on which the organic light emitting elements are to be made in an active matrix drive light emitting device. If insulating layers using organic resin materials are appropriately applied as structural elements of the substrate, then it is necessary to reduce gas emissions from the organic resin materials. Further, nitrogen gas and argon gas introduced into the film formation chamber are purified at the supply gate.
On the other hand, the control of the amount of polymerization cannot be completely performed for cases of forming layers made from high molecular weight organic compounds, and therefore a range of molecular weights develops and the melting point cannot be non-ambiguously determined. In this case a dialysis method or a high speed liquid chromatography method is applied. In particular, as the dialysis method, an electric dialysis method is suitable in removing ionic impurities with good efficiency.
The concentration of oxygen, which can lead to decreases in brightness and degradation of electrode materials such as dark spots, is thus reduced by using means such as those discussed above.
One form of a structure for active matrix drive, in which each pixel in a pixel portion formed by the organic light emitting elements that are formed as shown above is controlled by active elements, has TFTs formed on a substrate, each TFT having a semiconductor film, a gate insulating film, and a gate electrode, and organic light emitting elements are formed on the TFTs. A glass substrate is typically used as the substrate, and a minute amount of an alkaline metal is contained in barium borosilicate glass or aluminum borosilicate glass. The semiconductor film is covered by silicon nitride and silicon oxynitride in order to prevent contamination by alkaline metals from the glass substrate on the lower side and from the organic light emitting elements on the upper side.
On the other hand, the organic light emitting elements, which are preferably formed on a level surface, are formed on a leveling film made from an organic resin material such as polyimide or acrylic. However, this type of organic resin film is hygroscopic. The organic light emitting elements, which degraded by oxygen and H2O are covered by silicon nitride, silicon oxynitride, and diamond-like carbon (DLC) which have a characteristic to barrier gases.
FIG. 12 is a diagram for explaining the concept of an active matrix drive light emitting device of the present invention. A TFT 1201 and an organic light emitting element 1202 are formed on the same substrate as structural elements of a light emitting device 1200. The structural elements of the TFT 1201 are elements such as a semiconductor film, a gate insulating film, and a gate electrode, and elements such as silicon, hydrogen, oxygen, and nitrogen are contained in the structural elements. In addition, there are also elements such as metals for forming the gate electrode. On the other hand, the organic light emitting element 1202 contains alkaline metals such as lithium in addition to the main structural element of the organic compound material, carbon as elements.
Silicon nitride or silicon oxynitride 1205 is formed as a blocking layer on the lower side of the TFT 1201 (a glass substrate 1203 side). Silicon oxynitride 1206 is formed as a protective film on the opposite side, the upper side of the TFT 1201. On the lower side of the organic light emitting element 1202 is a silicon nitride or silicon oxynitride 1207 formed as a protective layer. A DLC film is formed as a protective film on the upper side of the organic light emitting element 1202. An organic resin interlayer insulating film 1204 is formed between both of the TFT 1201 and the organic light emitting element 1202, and is united with both of them. Alkaline metals such as sodium, those most disliked by the TFT 1201, are blocked by the silicon nitride or silicon oxynitride 1205 and by the silicon oxynitride 1206. On the other hand, the organic light emitting element 1202 most dislikes oxygen and H2O, and therefore the silicon nitride or silicon oxynitride 1207 and the DLC film 1208 are formed in order to block oxygen and H2O. Further, they also function in order to prevent alkaline metal elements in the organic light emitting element 1202 from reaching the outside.
In order to satisfy reciprocal quality with respect to impurity contamination, degradation due to mutual contamination of impurities is prevented by cleverly combining insulating films that block oxygen and H2O and by forming insulating films in light emitting devices thus structured by combining TFTs and organic light emitting elements.
Note that, in this specification, the term light emitting device indicates general devices which use the light emitting materials described above. Further, modules in which a TAB (tape automated bonding) tape or a TCP (tape carrier package) is attached to an element having a layer containing the above-mentioned light emitting material between an anode and a cathode, modules in which a printed wiring substrate is formed on the tip of a TAB tape or a TCP, and modules in which an IC is mounted by a COG (chip on glass) method on the substrate on which the light emitting elements are formed, are all contained in the category of light emitting devices.
Further, the concentration of oxygen as an impurity element indicates the lowest concentration measurable by secondary ion mass spectroscopy (SIMS) in this specification.